The present invention relates to an air gun of the type which can release pressurized gas with explosive suddenness for use in, for example, seismic surveying, particularly though not exclusively at sea.
As used herein the term "seismic air gun" means such a gun comprising:
a body;
a firing chamber within the body for containing pressurized gas to be released on firing of the gun;
an operating chamber within the body also for containing pressurized gas; and
a shuttle movably accommodated in the body having two pistons inter-connected by a stem, the stem having a throttled bore for communicating pressure from one end of the shuttle to the other;
a firing seal at the firing chamber co-operating with a firing one of the pistons to close the firing chamber when the shuttle is in a firing position, and
an operating seal at the operating chamber co-operating with an operating one of the pistons to close the operating chamber when the shuttle is in a position for firing, the operating seal sealing a greater area of the operating piston to the body than the area of the firing piston sealed to the body for maintaining the pressure of pressurized gas in the operating chamber acting on the operating piston with a force greater than that of the pressure of the pressurized gas acting on the firing piston, whereby the shuttle is held in its firing position.
Normally, the body will comprise a plurality of parts secured together.
It should, however, be noted that the above seismic air gun components may be incorporated in a gun or other apparatus still within the definition but intended for other uses. For example guns in accordance with this invention may be used in air operated pile drivers, mining face impact tools, stamping tools and road impaction hammers. Similarly pressurized gases other than air may be used.
As soon as the biassed balance of the pressure forces acting on the two pistons of the shuttle is disturbed from a bias towards the firing position to a bias away from this position, the gun fires and the pressurized gas is released from the firing chamber.
An early description of such a gun is in British Patent Specification No. 1,090,363, in particular in FIGS. 4 and 9. This gun is fired by bleeding pressurized gas from the operating chamber to the side of the operating piston opposite from the operating chamber. A partition seal with the shuttle stem retains a build up of pressure on the opposite side of the operating piston, and the pressure balance on the shuttle is disturbed. Once the pressure balance has been changed sufficiently to move the shuttle, the operating seal is opened allowing a sudden increase in pressure behind the operating piston. This results in a sudden acceleration of the shuttle due to the pressure of the firing chamber gas and to firing of the gun.
Following the invention of British Patent Specification No. 1,090,363, seismic air guns have been widely used and conventionally are fired by the mentioned bleeding to the opposite side of the operating piston. A more recent example is that of British Patent No. 2,023,829. This conventional mode of disturbing the pressure balance is susceptible to unreliability. In this context it should be remembered that seismic air guns are normally fired every few seconds for days on end. Generally they are towed behind survey ships which are very expensive to operate. Thus a delay of even a short time to replace an unreliable gun can be very expensive.
Unreliability is usually caused by leakage past seals. For instance, leakage of pressurized gas through a solenoid valve for controllably bleeding gas to the opposite side of the operating piston can cause misfiring. In the extreme, such leakage can cause continuous cycling of the gun which fires automatically, recharges and fires again in a fault known as auto-firing.
Leakage past the operating seal itself can also cause auto-firing. There are other seals which can leak, for instance, it is conventional for the operating piston to be a separate unit secured to the stem of the shuttle with an incorporated seal. Some seismic air guns have many seals for instance the seismic air gun described in U.S. Pat. No. 4,271,924 has seven seals associated with its operating chamber, not counting those incorporated in its firing solenoid valve.
Apart from the operating seal, another important seal which is between moving parts is the partition seal which seals a partition to the stem interconnecting the shuttle pistons, the partition extending across the body of the gun transverse to the stem, which passes through the partition. If the partition seal is worn there may be an insufficient build-up of pressure on the opposite side of the operating piston for the gun to fire at all when the firing solenoid valve is actuated. Erratic sealing of this partition seal or any seal involved with the transfer of air across the operating piston for the purpose of firing the gun, will cause a variation in firing time lag. If excessive the gun will be rendered useless in a synchronised array.
It is an object of this invention to provide a simpler seismic air gun firing by a simpler method.